This invention relates generally to a new composition of matter and more specifically to improved toner compositions for use in flash fusing in electrophotographic imaging processes.
The formation and development of images on the surface of electrophotographic materials by electrostatic means is well known and basically involves subjecting a xerographic plate comprising a conductive backing upon which is placed a photoconductive insulating surface to a uniform charge and subsequently exposing the photoconductive surface to a light image of the original to be reproduced. The photoconductive surface is prepared in such a manner so as to cause it to become conductive under the influence of the light image in order that the electrostatic charge formed thereon can be selectively dissipated to produce what is developed by means of a variety of pigmented resin materials specifically made for this purpose such as toner. The toner material is electrostatically attracted to the latent image areas on the plate in proportion to the charge concentration contained thereon thus areas of high charge of concentration become areas of high toner density and correspondingly low charge images become proportionally less dense. Thereafter the developed image is transferred to a final support material such as paper and affixed thereto for a permanent record or copy of the original.
Numerous methods are known for applying the electroscopic particles to the electrostatic latent image to be developed, such as for example cascade development as described in U.S. Pat. No. 2,618,552; magnetic brush as described in U.S. Pat. Nos. 2,874,063; 3,251,706; and 3,347,402; powder cloud development as disclosed in U.S. Pat. No. 2,217,776; and touchdown development as described in U.S. Pat. No. 3,166,432.
The image can be fixed by a number of various well known techniques including for example vapor fixing, heat fixing, pressure fixing or combinations thereof as described for example in U.S. Pat. No. 3,539,161. These techniques of fixing do suffer from some deficiencies thereby rendering their use either impractical or difficult for certain electrophotographic applications. It has been found for example rather difficult to construct an entirely satisfactory heat fuser which has high efficiency, ease of control and short warm-up time. Also heat fusers sometimes burn or scorch the support material, for example the paper. Somewhat similar problems exist with pressure fixing methods whether used with heat or without heat and more particularly such problems include for example image offsetting, resolution degradation, and further additionally there cannot be consistently produced a good permanent type of fix. Vapor fixing has several advantages but one of its main problems is that the toxic solvent that is used in many cases makes such a method commercially unattractive because of the health hazards and pollution control standards involved. For example equipment and apparatus to sufficiently isolate the fuser from the surrounding air is very complex, costly, difficult to operate and thus it is difficult to obtain consistent results in such situations.
Many of the modern electrostatographic reproducing apparatus resulted in the development of new materials and new processing techniques, one such apparatus being an automatic electrostatographic reproducing apparatus which is capable of producing copies at an extremely rapid rate. In such situations it appears that the best method for fixing is radiant flash fusing, one of the main advantages of such a technique being the energy which is emitted in the form of electromagnetic waves is available immediately and requires no intervening medium for its propagation. However, although an extremely rapid transfer of energy between the source and the receiving body is provided when using the flash fusing process, a problem encountered with such a system is obtaining an apparatus which can fully and efficiently utilize a preponderance of the radiant energy emitted by the source during a relatively short flash. The toner image usually comprises a relatively small percentage of the total area of the copy receiving the radiant energy and in view of this the properties of most copying materials as for example paper, causes most of the energy to be wasted as it is transmitted to the copy or is reflected away from the fusing area.
Additionally, when radiant energy from the flash fuser is generated at high levels which is necessary in order to fuser the toner, objectionable odor and smoke results in some instances because of the thermal decomposition of the base resin at the temperature at which fusing must occur. Further additives have been employed in the prior art in an attempt to eliminate such decomposition but this has not been successful and further in many instances the additives being used decompose under the process conditions of development.
The flash energy used in a flash fusible toner system is absorbed in a layer of toner of finite thickness adjoining the outer toner surface with the absorption being greatest at the surface and constantly decreasing with increasing distance from the outer surface. The flash is of very short duration on the order of about 1 millisecond and consequently the toner very close to the surface is heated to a much higher temperature than the toner mass as a whole, thus in view of the higher temperature the majority of the decomposition that occurs takes place very close to the toner surface. Additionally, the volatile material that is formed cannot be absorbed or entrapped by the decomposed toner matrix and thus it escapes from the toner layer before the toner cools appreciably. Thus the volatile decomposition products formed close to the surface have a much higher probability of escaping as effluents than do those formed deep inside the toner layer. This, together with the greater decomposition close to the surface as compared to the decomposition occuring inside the toner mass, causes the undesirable decomposition products to arise almost entirely in a very thin layer of toner next to the surface. Thus there is need for a material that will eliminate and/or substantially control this decomposition while at the same time being compatible with the toner composition itself, such additive not effecting the system in any other adverse manner. Such an additive should also be able to withstand decomposition itself.
At the same time there is a need for materials such as plasticizers which lower the viscosity of the toner resin, are nonvolatile and thermally stable. It is well known that toner is subjected to mechanical attrition which tends to break down the particles into undesirable dust fines and such fines are detrimental to machine operation in that they are extremely difficult to remove from reusable imaging surfaces and also because they tend to drift to other parts of the machine and deposit on critical machine parts such as optical lenses. The formation of these fines is reduced somewhat when the toner contains a tough high molecular weight resin which is capable of withstanding the shear and impact forces imparted to the toner during the development process. However, unfortunately many high molecular weight materials cannot be employed in high speed automatic machines as they cannot be rapidly fused during a powder image heat fixing step. In order to avoid combustion additional equipment such as complex and expensive cooling units are necessary to properly eliminate the large quantity of heat generated in the fuser. For this reason it is important to lower the point at which the toner flows so that less energy can be used to cause it to perform properly.